2,5-dichloro-3-amino-benzoic acid is an important selective herbicide, and is usually prepared by the reduction of the corresponding 3-nitro compound. However, it has proved difficult to prepare 2,5-dichloro-3-nitro-benzoic acid (itself a selective herbicide) to an acceptably high degree of purity, because the conventional preparations also give rise to other unwanted nitro isomers, which have to be separated from the desired product.
A widely used preparative route involves the nitration of 2,5-dichloro-benzoic acid or a derivative thereof, and whilst this yields the desired 3-nitro compound, significant amounts of the unwanted 6-nitro isomer are also formed and must be separated from the desired product. The separation of the unwanted isomer from the 3-nitro compound is a difficult, and therefore expensive, procedure.
This invention concerns new compounds, 2,5-dichloro-3-nitro-terephthalic acid and certain esters thereof, and a novel process for the preparation of 2,5-dichloro-3-nitro-benzoic acid involving these compounds, which avoids the problems of conventional processes.
In two articles dated 1888 (Berichte 21, 1467-1468 and 1959-1964), Levy and Andreocci describe processes purporting to prepare, amongst other products, 2,5-dichloro-3-nitro-terephthalic acid and the dimethyl ester thereof. However, the physical and chemical properties of the products of their processes differ markedly from those of the relevant compounds prepared by methods described herein. The compounds that Levy and Andreocci prepared were not what they alleged them to be, but more probably 2,5-dichloro-3,6-dihydro-3-nitro-terephthalic acid and its dimethyl ester.